This should do all of the work to resolve #1182.

There should be no breaking changes because of this work.

In most cases this could be made to work async via a new determineUsername() function which uses .getUsername() internally. All authentication mangers now extend a base class which impliments username fetching and caching.

Some cases like getSharingUrl() or isItemOwner() which are public synchronous methods cannot be modified to be async so tey where modified to accept a username param as a second argument which uses getUsername() in the async functions that use them. These should still work as implemented if people continue to use them with ArcGISIdentityManager but might not work with API keys.

In our next breaking change we should:

1. Remove the synchronous username entirely to force use of getUsername()
2. Add new options for initiating API key and app auth in an async way to pre-cache the username.